Drilling for natural resources such as petroleum and natural gas can include advancing a borehole for many miles to reach underground deposits. In a drilling operation, a drill bit is mounted to the end of a drill string. The drill string is rotated from the top of the string or by a motor at the bottom of the string, or both, to rotate the drill hit and advance the borehole. FIG. 1 is a schematic representation of a drilling operation 2. In conventional drilling operations a drill bit 8 is mounted on the end of a drill string 6 comprising drill pipe and drill collars. The drill string may be several miles long and the bit is rotated to advance the borehole by failing strata ahead of the bit. A pump circulates drilling fluid through the drill pipe and out of the drill bit flushing rock cuttings from the bit and transporting them back up the wellbore. The drill string 6 comprises sections of pipe that are threaded together at their ends to create a pipe of sufficient length to reach the bottom of the wellbore. Tools 10 such as motors and measurement while drilling (MWD) equipment can be incorporated in the drill string as well.
Positive displacement or mud motors (PDM) can be included in the drill string to convert the pressure of drilling fluid pumped down the drill string to rotational motion. The rotational motion or torque can be used for various purposes including creating transverse vibration in the drill string or rotating the drill bit. A mechanical link connects the mud motor to the bit. The mechanical link can be a universal joint that converts eccentric rotation of the mud motor output to axial rotation.
Variations in the pressure of the drilling fluid that drives the mud motor can create surges in the output torque of the mud motor and unstable rotation of the rotor in the housing causing damage to the rotor and/or stator. Where the mud motor is linked to a drill bit, variations in drag on the bit in the borehole can result in stick-slip rotation which can generate high torque or surging torque that can damage the mud motor and the link. This can result in unscheduled servicing of the drill string and significant expense.
A downhole tool that limits the torque in a mechanical link between connected tools to limit damage would be advantageous.